The invention relates to frequency converters. More particularly, the invention relates to a frequency converter arrangement with at least two successive heterodyne stages, a first heterodyne stage which converts an input signal with an adjustable first heterodyne frequency of a first superposition oscillator, and a second heterodyne stage which converts an input signal with a fixed second heterodyne frequency of a second superposition oscillator into a second intermediate frequency.
Frequency arrangements of this type are known; see, e.g., German patent documents Nos. DE 27 44 432 and DE 40 21 294 and Japanese Patent Abstracts 60-28330A and 61-252720A. In such arrangements, a frequency divider which can be set in integral division ratios is used as frequency divider. The reference oscillator is an oscillator that is tuned to a fixed frequency. The phase comparison takes place at a frequency that is obtained by dividing the frequency of the reference oscillator. This frequency determines the step width with which the receive frequency can be shifted. To achieve a step width of 25 kHz, for example, just such a reference frequency is required. Due to this low reference frequency, the multiplication factor is large and the maximum possible control bandwidth is small, which leads to relatively bad phase noise and high transient recovery times in the frequency change. Thus, these known arrangements are not suitable for frequency conversion arrangements in receivers or spectrum analyzers with a frequency resolution in the Hz range.
The state of the art in these devices involves the mixing of the first local oscillator with the harmonic of a good frequency reference and the synchronizing of the mixed signal with a synthesizer signal. Due to the independent synchronization of the first and second local oscillators, the phase noise of these two oscillators is added, whereby the first oscillator dominates, since it has the higher frequency and is tuned via a synthesizer. The difference of the phase noise between the highest receive frequency and a receive frequency near zero is approximately 6 dB, since the first local oscillator is typically tuned over an octave and the phase noise is determined by the multiplication factor to the reference. This is not sufficient for applications in the receive range of up to several GHz; a phase noise is expected which is proportional to the receive frequency.
The same is true for signal generators in which the output frequency is generated in two successive heterodyne stages with a fixed, or respectively, a variable heterodyne frequency (or: beat frequency), as is the case in modulable signal generators.
It is an object of the invention to provide a frequency conversion arrangement which is suitable for a receiver, spectrum analyzers, or respectively, signal generators and which has minimal phase noise even at high frequencies.
To that end, in an embodiment the invention provides a frequency conversion arrangement with at least two successive heterodyne stages, the first heterodyne stage which converts an input signal with an adjustable first heterodyne frequency of a first superposition oscillator, which can be set in fine frequency increments, into a first intermediate frequency, and a second heterodyne stage which converts an input signal with a fixed second heterodyne frequency of a second superposition oscillator into a second intermediate frequency, the difference frequency between the first and second heterodyne frequencies being converted with an adjustable frequency divider into a lower frequency, which is compared in a phase detector to the output frequency of a reference oscillator, and the first superposition oscillator being synchronized with the output frequency of the reference oscillator via a phase control loop with the output signal of this phase detector, characterized in that the reference oscillator is a synthesizer that can be set in fine frequency increments.
In an embodiment, the invention provides a frequency conversion arrangement characterized in that the adjustable frequency divider is a fractional N-divider.
In an embodiment, the invention provides a frequency conversion arrangement characterized by its application in a high-frequency receiver or spectrum analyzer, whereby, in a first heterodyne stage (superposition oscillator) with a heterodyne frequency that can be set in fine frequency increments, the input signal to be received is converted into a first constant intermediate frequency, which is subsequently converted with a fixed second heterodyne stage (heterodyne frequency) into a lower second intermediate frequency.
In an embodiment, the invention provides a frequency conversion arrangement characterized by its application in a high-frequency receiver or spectrum analyzer, whereby, in a first heterodyne stage (superposition oscillator) with a heterodyne frequency that can be set in fine frequency increments, the input signal to be received is converted into a first constant intermediate frequency, which is subsequently converted with a fixed second heterodyne stage (heterodyne frequency) into a lower second intermediate frequency.
In accordance with the invention, a synthesizer which can be set in fine frequency increments is used instead of a reference oscillator with a fixed frequency. The reference frequency which is fed to the phase detector can thus be so set that the difference between the reference signal and the signal generated by the frequency divider has minimal noise.
It has proven particularly advantageous to implement the frequency divider as a fractional N-divider with single stage integration, because the division ratio of the frequency divider can thus be set to a minimal division factor and thus to minimal noise. A frequency conversion arrangement is thus inventively created which has minimal phase noise.
It is also conceivable to control to the sum frequency of the two superposition oscillators (or: harmonic oscillators) instead of to the difference frequency, according to how the frequency positions of the frequency conversions are intended.
The same principle can be applied both in a receiver or spectrum analyzer and in a high-frequency generator. In both cases, the sum of the phase noise is appreciably improved compared to conventional frequency conversion concepts. For example, given small receive frequencies, the phase noise is about 20 dB better than in a known arrangement; in the highest receive frequency, up to 6 dB better.
These and other features of the invention are discussed in greater detail below in the following detailed description of the presently preferred embodiments with reference to the accompanying drawings.